Mass spectrometers are devices for making precise determinations of the constituents of a material by providing separations of all the different masses in a sample according to their mass to charge ratio. The material to be analyzed is first disassociated/fragmented into charged atoms or molecularly bound groups of atoms, i.e. ions.
There are several distinct types of mass spectrometers. The quadrupole mass spectrometer is a relatively recent apparatus which was first described in a paper by Paul, et al. in 1952. The quadrupole mass spectrometer differs from earlier spectrometers because it does not require use of large magnets but employs radio frequency fields in conjunction with a specifically shaped electrode structure. In this structure, RF fields can be shaped so that they interact with ions so that the resultant force on certain ions is a restoring force so that the ions are caused to oscillate about a neutral position.
In the quadrupole mass spectrometer (QMS), four, long, parallel electrodes, each having precise hyperbolic cross sections, are connected together electrically. DC voltage, U, and RF voltage, V.sub.o cos Wt can be applied to the electrodes. In the QMS, restoration forces act on the ions in two directions only, so the trapped ions travel with a constant velocity down the axis as they oscillate around the axis.
Another closely related device also disclosed in the Paul, et al., paper has become known as the quadrupole ion trap (QIT). The QIT is capable of providing restoring forces to the ion in all three directions and can actually trap ions of selected mass/charge ratio. The ions so trapped are capable of being retained for relatively long periods of time which supports separation of selected masses and important scientific experiments and industrial testing which is not as convenient to accomplish in other spectrometers.
Only in very recent years has the QIT become of increased importance as a result of the development of relatively convenient techniques for ionizing, trapping, isolating and separating trapped ions. Ionization is usually by electron bombardment. By adjusting the QIT parameters so that it stores only a selectable range of ions from the sample within the QIT, and then linearly changing, i.e. scanning one of the QIT parameters, it is possible to cause consecutive values of mass/charge (m/z) of the stored ions to become successively unstable. This is called the instability scanning mode, as disclosed in U.S. Pat. No. 4,540,884. The mass spectrum of the trapped ions is obtained by sensing the intensity of the unstable ions which provide a detected ion current signal as a function of the scan parameter.
The QIT has also become very useful in a new mass spectrometer technique known as MS/MS where a selected ion is retained in the QIT and all the other trapped ions are ejected; then the remaining ion or ions (parent) are disassociated and the fragments (daughter ions) are scanned out of the trap to obtain the mass spectrum of the daughter ions.
The MS/MS technique requires improved ion isolation. Isolation techniques have been improved by use of so called "supplementary generators" to assist in the selective isolation of particular ions by resonantly ejecting unwanted ions. U.S. Pat. No. 4,749,860 employs such a supplemental generator RF field which is connected across the QIT end caps and provides an excitation frequency which corresponds to the so called "secular frequency" of an ion which is to be ejected. For example, to isolate an ion m(p), the supplemental frequency can be selected, for a particular RF trapping voltage, to be equal to the secular frequency of the next closest trapped ion having m/z ratio of m(p)+1. The supplemental voltage is applied to the end caps of the trap simultaneously with the scanning of the voltage of the trapping field. This approach suffers from at least three problems. First, mass instability scanning to eject ions of mass less than m(p) suffers from poor mass resolution and thus results in significant loss in the intensity of the m(p) ion while attempting to completely remove the m(p)-1 ion out of the stability region. Second, the stability boundary on the high side is flat so that this procedure also suffers significant loss of the m(p) ion when trying to eliminate the m(p)+1 ion. Finally, it is essential to know the precise value of the voltage of the RF trapping field. To calculate the precise secular frequency, it is probably impossible to know the exact voltage acting on the ions because of the mechanical or electrical (electrode) imperfections and because of space charge effects which act to shift the stability region significantly. The so called space charge effect is known to significantly effect the secular frequency. The equation which defines the secular frequency is ##EQU1## where W.sub.o is the RF trapping field frequency and W is the secular frequency at any value of .beta..sub.z. It has become the practice to apply the supplemental frequency to eject the higher m(p)+1 ions at low values of .beta..sub.z because the relationship between .beta..sub.z and the other stability parameters outside this region is non-linear and the resolution at usual scan speed is poor. Also, at lower RF trapping field voltage, the average ion energy is lower and ions can be created and retained in the trap more efficiently, other parameters being equal. Furthermore, there is a limit to the maximum mass which can be ejected by this technique unless the value of the RF field is increased. The '860 patent, to eject the higher masses, adds the additional step of frequency scanning the supplemental frequency down to low frequencies which requires complex equipment and introduces undesirable additional isolation process steps.
It is known to employ broadband supplemental waveform generators such as a Fourier Transform (FT) synthesizer to create a time domain excitation based on a spectrum of desired excitation frequencies to cause tailored ejection of specific bands or ranges of ions. As pointed out in U.S. Pat. No. 4,761,545, the FT synthesizer technique employs very high power amplifiers. Also, even when phase scramblers are used with FT, it is not possible to achieve arbitrary excitation frequency spectrum at suitable low peak excitation voltages because of so called Gibbs oscillations.
It is also known from European Patent Application, EPO 362432A1, to shorten the process scan time in a QIT by simultaneously eliminating uninteresting ions at the same time off their creation. The express reason for the procedure is stated in this EPO patent, at Col. 4, line 7, "The advantage of this method is the shorter time needed to eliminate the unwanted ions as compared to . . . alternate steps . . . ".
The McLuckey paper, J. Am. Soc. Mass Spectrometer, 1991, V. 2, p. 11-21 recognizes that situations can occur where desired ion accumulation cannot occur due to rapid buildup of matrix ions, and that matrix ion ejection might be most useful when applied during ion accumulation. Although McLuckey noted empirically seeing discrimination effects of space charge in situations of widely different m/z values, he did not disclose or identify the relationship between space charge and stored mass or the significance of the effects of common environmental air gases on the accumulation of high m ions.